I agree it sounds like a bacterial infection. One thing that wasn't talked about was conditions in your tank. Fish often get sick when their water quality is poor, so it is iportant that you get your water checked. Also, what fish are in your tank, and how big of a tank is it?
When you go to the LFS, you need to look for an antibacterial medicine. I would treat with Kanacyn, tetracycline, or Maracyn AND maracyn 2.
Here is some more info for you. I am not the author of this information.
Ulcer Disease
Symptoms:
Fishes presenting with large red open lesions, ulcers or sores on the body have ulcer disease. Other symptoms may also include eroding flesh, reddening at the base of the fins and vent, and the fish may also experience a loss of appetite and darkening of color.
Cause:
Internal bacterial infection caused by Aeromonas and Pseudomonas. This disease is most often seen in fish that have been kept under poor conditions or recently imported.
Treatment:
Check your water conditions to ensure that they are within the optimal ranges for the type of fish you are keeping. If not, perform an immediate water change. Ulcer Disease is highly contagious, therefore the entire tank and all fish, even those not yet showing symptoms, should be treated. Treatment with Kanacyn, Furacyn, or Nitrofura-G are recommended.
